It's very difficult to list all the beers Magnolia has on tap at any given time.  And with the amount they have (up to 12) there is a lot of turnover, allowing for the entire spectrum of ale styles.

Most of the beers are standard beers.  They have a few seasonals, but not really any specialty brews.  But with the amount of regular beers, that's OK!

Here are the standard beers:

Prescription Pale - All cascade hopped, not-so-pale ale.  It's more of an amber color, but has the properties of a pale.  The great Crisp floor malted grains stand up beautifully to the nice hop character.

Blue Bell Bitter - Not so much a bitter as an ESB, Blue Bell is very British.  The British malts and subtle hops provide a great malty taste.

Spud Boy's IPA - A great IPA that starts off in a British style, but ends American with the late addition of Cascade hops.  And at 6.4%, Spud Boy actually makes a great session beer for the hop lovers out there.

Sara's Ruby Mild - A bit stronger than a British mild, Sara's has a very nice balance between the British malt and hops.

Big Cypress Brown - Formerly known as Willie's Brown.  Nice British style brown.

Cole Porter - Robust porter with nice black malt character.

Stout of Circumstance - One of my favorite Oatmeal Stouts ever.  After having many bad stouts over the years at various pubs, Stout of Circumstance was such a welcome stout.  It has a great, smooth taste along with a perfect balance of roasted barley and other dark grains.  The perfect breakfast beer, finishing beer, or anywhere in between.

Proving Ground Pale - This one is brewed exclusively for the hop heads of the world.  It has a calculated 100 IBU's!  So hoppy it almost makes a hop head's face pucker.

Weather Report Wheat - A light, refreshing, unfiltered wheat that's seasoned with a hint of coriander.

Ashbury Alt - Probably the only alt available in San Francisco.  It's an amber-ish type of ale hopped with German hops to give it a very distinct taste.

Some seasonals:

Spooky Tooth Stout - A dry Irish stout served on nitrogen.  A little more roasty than most Irish stouts.

Winter Warmer - Kind of a strong ale type of beer.  It seems to come out a little different every year.  This year's had strong cherry esters.

Old and in the Ale - Brewed in the summer in the tradition as the Winter Warmer.

Jubilee Ale - Brewed for the Haight Street Fair in June every year.  The beer is poured by vendors up and down the street and at the pub.  Similar to Prescription in character.

Old Thunderpussy Barley Wine - Many Magnolia die-hards wait and wait for the arrival of the barley wine.  Mangolia has been in the top 8 at the Toronado Barley Wine Festival.

Magnolia's site has very accurate description of their beers, so here's the link.